Aluminium Design and Construction - Chapter 2 potx

CHAPTER 2Manufacture In this chapter we describe the manufacture of wrought aluminium products, such as plate, sheet, sections and tube, known in the trade as semi-fabricated products or

CHAPTER 2

Manufacture

In this chapter we describe the manufacture of wrought aluminium products, such as plate, sheet, sections and tube, known in the trade

as semi-fabricated products (or ‘semi-fabs’) The two main stages are

production of pure aluminium ingot, and conversion of this into wrought material

2.1 PRODUCTION OF ALUMINIUM METAL

2.1.1 Primary production

The production of aluminium ingot comprises three steps: (1) mining the bauxite ore; (2) extracting alumina therefrom; and (3) smelting Bauxite is plentiful in many countries and is extracted by opencast mining Alumina (A12O3), a white powder, is obtained from bauxite by the Bayer process which requires a large supply of coal and caustic soda Roughly 2 kg of bauxite, 2 kg of coal and 0.5 kg of caustic soda are needed to produce 1 kg of alumina

In the smelting operation, metallic aluminium is extracted from alumina

by electolysis, using the Hall-Héroult process It takes place in a cell (or

‘pot’) comprising a bath of molten cryolite (the electrolyte) and carbon electrodes A typical pot is around 4 or 5 m long The cathode covers the floor of the bath, while the anodes are in the form of massive carbon blocks which are gradually lowered into the cryolite as they burn away

Alumina is fed in at the top of the molten pool, where it dissolves, and molten aluminium is drawn off from the bottom The metal emerges

at a temperature of around 900°C and at a high purity (99.5–99.8%) Copious fumes are produced The electric supply is direct current at a typical potential of 5 V per pot, the current requirement being high, around 100–150 kA Continual replacement of the carbon anodes is also

a significant factor The production of one kilo of aluminium consumes roughly 2 kg of alumina, 0.5 kg of carbon and 15 kWh of electrical energy The cryolite (Na3AlF6) is largely unconsumed

The main requirements for the production of aluminium are thus bauxite, coal and cheap electricity These are never all found in one place The location usually preferred for a smelter is near a dedicated hydroelectric power plant, which may be thousands of kilometres from the alumina source and from markets for the ingot, as, for example, with the Kitemat smelter in British Columbia, Canada Sometimes a smelter is designed for strategic reasons to run on coal-fired electricity The new 50 000 tonne/yr smelter at Richards Bay in Natal, South Africa, which relies on coal-fired electricity, has a total of nearly 600 pots contained

in four ‘potrooms’ each 900 m long×30 m wide

2.1.2 Secondary metal

Not all the metal going into aluminium products comes from ingot, an important ingredient being ‘secondary metal’, i.e scrap This is partly supplied by scrap merchants, and partly comes from process scrap generated in the rolling mill or extrusion plant The composition of such scrap is important, the best scrap being pure aluminium or a low alloy Melted-down airframe material is less convenient, as it contains

a relatively large amount of copper or zinc, making it less suitable for making non-aeronautical alloys

2.2 FLAT PRODUCTS

2.2.1 Rolling mill practice

Aluminium plate and sheet are manufactured in conventional rolling mills, the main difference from steel being the lower temperatures involved

Alloyed metal is produced by melting a mixture of ingot and scrap,

to which are added metered quantities of ‘hardeners’ (small aluminium

ingots containing a high concentration of alloying ingredient) Rolling

slabs are then produced by vertical continuous casting in a long length,

from which individual slabs are cut Each slab is skimmed on both faces and slowly heated in a furnace, from which it enters the hot-line This might typically comprise a hot mill followed by a three-stand tandem set-up The material emerging therefrom, called ‘hot-mill strip’, can be shipped directly as plate or reduced further in a cold mill to produce sheet The immediate output from the cold mill is coiled strip, which is decoiled and cut into sheets Plate and sheet widths thus produced are typically about 1.5 m, although greater sizes are possible

2.2.2 Plate

The term ‘plate’ refers to hot-rolled flat material, typically with a thickness

⭓ 6 mm, although it may be thinner When in a non-heat-treatable alloy, plate is often supplied in the rather vague ‘as-hot-rolled’ or F condition, for which only typical properties can be quoted Alternatively

it may be annealed after rolling, bringing it to the O condition, in which case the strength is lower but clearly specified It is also possible for non-heat-treatable plate to be rolled to a temper (i.e specific hardness), although this can be difficult to control on the hot-line

Heat-treatable plate (2xxx, 6xxx or 7xxx-series alloy) is usually supplied

in the fully-heat-treated T6-condition, which involves a quenching process followed by artificial ageing Alternatively it can be produced in the more ductile T4-condition, when it is allowed to age naturally at room temperature

2.2.3 Sheet

By ‘sheet’ we generally refer to flat material up to 6 mm thick (although

in the USA the term ‘shate’ is sometimes used for material on the border-line between sheet and plate) It is produced by cold reduction, which usually involves several passes with interpass annealing Readily available thicknesses are 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 and 6.0 mm Most sheet is produced in non-heat-treatable material (1xxx, 3xxx,

5xxx), and is supplied in a stated temper (such as ‘quarter-hard’,

‘half-hard’) with specified mechanical properties These are achieved by a controlled sequence of rolling reductions, possibly with interpass

annealing The sheet may be temper rolled, in which case it is given a precise reduction after the last anneal Or it may be temper annealed,

when the final stage is a suitably adjusted anneal after the last rolling pass

For more highly stressed applications, sheet is supplied in heat-treatable alloy (2xxx, 6xxx or 7xxx series), usually in the fully-heat-treated T6-condition (quenched and artificially aged)

2.2.4 Tolerance on thickness

The thickness tolerance dt for flat products allows for variation across

the width, as well as for inaccuracy in the main thickness Its value

depends on the thickness t and width w, and may be estimated

approximately from the following expressions:

Plate dt=±(0.0000 14 wt+0.3) mm (2.1) Sheet dt=±(0.11Öt+0.00004 w–0.06) mm (2.2)

in which t and w are in mm These agree reasonably with the BSEN.485 requirements for t=1–30 mm and w > 1000 mm Outside these ranges,

they overestimate the tolerance

2.2.5 Special forms of flat product

(a) Clad sheet

Sheet material in 2xxx-series alloy, and also in the stronger type of 7xxx, can be produced in a form having improved corrosion resistance

by cladding it with a more durable layer on each surface Such a product

is achieved by inserting plates of the cladding material top and bottom

of the slab as it enters the hot-mill As rolling proceeds, these weld on and are steadily reduced, along with the core, the proportion of the total thickness being about 5% per face For 2xxx-series sheet, the cladding

is in pure aluminium, while for the 7xxx an Al–1% Zn alloy is preferred

(b) Treadplate

Aluminium treadplate is available with an anti-slip pattern rolled into one surface, as in steel It is normally supplied in the stronger type of 6xxx-series alloy in the T6-condition

(c) Profiled sheeting

This product, which can be used for many sorts of cladding, is made by roll-forming An important use is for the cladding of buildings, where

a trapezoidal profile is usually specified, formed from hard-rolled sheet

in 3xxx-series alloy Some such profiles are of ingenious ‘secret-fix’ design

(d) Embossed sheet

This is sheet having a degree of roughening (of random pattern) rolled into one surface It can be used in order to reduce glare It is also claimed to improve the stiffness slightly Such sheet is sometimes employed

in the manufacture of profiled sheeting

(e) Cold-rolled sections

As in steel, it is possible to produce small sections by roll-forming from strip However, the technique is not generally favoured for aluminium, because extrusion has more advantages But it comes into its own for very thin sections, of thickness less than the minimum extrudable, say,

1 mm and under

2.3 EXTRUDED SECTIONS

2.3.1 Extrusion process

Although available for some other non-ferrous metals, such as brass and bronze, it is with aluminium that the extrusion process has become

a major manufacturing method, far more so than with any other metal [7] This is partly due to the relatively low temperature at which the metal will extrude (roughly 500°C)

The process enables aluminium sections to be produced from 10 to

800 mm wide, with an unlimited range of possible shapes The tool cost for producing a new section is a minute fraction of that for a new section

in steel, as it merely involves cutting a new die Also, the down-time at the press for a die change is negligible compared to the time lost for a roll change in the steel mill In aluminium, it is therefore common practice

to design a special dedicated section to suit the job in hand, or a ‘suite’

of such sections, and the quantities do not have to be astronomic to make this worthwhile An important feature of aluminium extrusion is the ability to produce sections that are very thin relative to their overall size There are various versions of the extrusion process Aluminium sections

are normally produced by direct extrusion As for flat products, the starting

point is molten metal with a composition carefully controlled by the addition of hardeners Long cylindrical ‘logs’ are then produced by continuous casting, often at the smelting plant These are cut into shorter lengths by the extruder to produce the actual extrusion billets Previously these were always skimmed in a lathe, to remove surface roughness and impurities, but modern practice is to dispense with this in the case

of 6xxx-series billets, because of improvements in log casting Each billet is preheated in an induction furnace and then inserted in the heated container of the extrusion press (Figure 2.1) The hydraulic ram

at the back of the billet is then actuated, causing the metal at the front end to extrude through the die and travel down the run-out table The aperture in the die defines the shape of the emerging section The process

is continued until some 85–90% of the billet has been used The as-extruded length may reach 40 m

The extrusion ratio is the ratio of the section area of the billet to that

of the section extruded For 6xxx-series alloys, it ideally lies in the range 30–50 Too low a value (say, 7 or less) will cause a drop in properties; while too high a value (say, 80 or more) means an excessive ram pressure, with the possibility of die distortion and breakage

Extrusion presses vary greatly in size, with the container bore (billet diameter) ranging from about 100 to 700 mm The required pressure from the ram depends on the alloy and the extrusion ratio Presses are rated according to their available ram force, which typically lies between 10 and

120 MN (1000 and 12 000 tonnes) Although presses are mostly located in

big extrusion plants, it is not uncommon to find a small press in the factory of a specialist fabricator, such as a metal window firm

2.3.2 Heat-treatment of extrusions

Most extrusions are produced in heat-treatable material, and to bring them up to strength they have to undergo solution treatment (quenching) followed by ageing

The easiest form of solution treatment is simply to spray the section with water as it emerges from the press, and this is the usual procedure for thinner sections in 6xxx-series alloy With some 6xxx material, a useful degree of hardening is even achieved with the spray switched off (‘air-quenching’), thereby reducing distortion

Quenching at the press is less effective with thick 6xxx material, and with the 2xxx and 7xxx-series alloys it is no good at all, because these require precise control of the solution treatment temperature For such material, it is necessary, after cutting into lengths, to reheat and quench

in a tank This can be done vertically or horizontally The former causes less distortion, but imposes a tighter limitation on length

For most extruded material, the second stage of heat treatment (the ageing) consists of holding it in a furnace for some hours at an elevated

Figure 2.1 Extrusion process (direct extrusion).

temperature somewhere in the range 150–180°C This is known as artificial

ageing or precipitation treatment, and takes the metal up to its full strength

T6 condition (or T5 if air-quenched) It is performed after correction Sometimes the quenched material is left to age naturally at room

temperature (natural ageing), bringing it to the more ductile but weaker

T4 condition This would be preferred for material that has later to go through a forming operation

2.3.3 Correction

Extrusions tend to distort as they come off the press, and the quenching operation makes this worse There are basically two forms of distortion that have to be corrected: (a) overall bow along the length; and (b) distortion of the cross-section

Overall bow is got rid of by stretching, typically up to a strain of 1 or 2% For spray or air-quenched material, and also for non-heat-treated, stretching can take place on the full length of the section as extruded, before it is cut into shorter lengths For other materials, it has to be done length by length after quenching For heat-treated extrusions, the stretch has little effect on the final material properties But for non-heat-treatable extrusions, it has the effect of significantly lowering the compressive proof stress (the Bauschinger effect), a fact that is apt to be ignored by designers For thick compact sections, distortion of the cross-section is no problem, and the only correction they need is the stretch But for thin slender ones this form of distortion can be serious and further correction is needed Various techniques are available, expecially roller correction, and these are tailored to suit the profile concerned These techniques tend to be labour intensive and, for this reason, slender sections cost more per kilogram Sometimes the likelihood of serious distortion in a very slender profile will make a proposed section impracticable, even though it can be extruded In such cases, a possible answer may be to reduce the distortion by specifying the air-quenched T5 condition (instead

of T6), provided the lower properties are acceptable

2.3.4 Dies

Extrusion dies, which are made by the thousand, are in a special hard heat-resisting steel, the aperture being machined by spark erosion The tooling cost for a straightforward structural die (non-hollow), 150 mm wide and without complications, might be roughly one-third of the cost

of a tonne of the metal supplied from it

Skill is needed to make a die so that the section produced comes out more or less straight and level down the run-out table Referring to Figure 2.2, it is the aperture dimension at the entry side that controls the thickness The thick parts of a section tend to extrude faster than

the thin parts, so that a section such as the one shown would come out

in a curve if nothing were done To counter this effect, the die designer retards the flow of metal in the thick regions by increasing the depth

of ‘land’ (dimension x) Even so, a section never comes off the press

completely straight and subsequent stretching is always needed The performance of a die can be improved if any re-entrant corners

in the aperture (outside corners on the section) are slightly radiused, even with a radius of only 0.3 mm It is bad practice to call for absolutely sharp corners unless these are essential They increase the risk of die failure and reduce the permitted extrusion speed

The pressure acting on the face of a die during extrusion is very high, possibly approaching 700 N/mm2 With sections such as those shown in Figure 2.3, there is the possibility that the die will break along line Y due to the pressure acting on region X This tendency depends

on the aspect ratio (a) of the region X, defined by:

(2.3a)

(2.3b)

where c, d are as defined in the figure and A is the area of region X.

Under the most favourable conditions, i.e for a section in 6063 alloy (or

Figure 2.2 Typical extrusion die.

Figure 2.3 Re-entrants in extruded sections.

pure aluminium) having rounded corners at the tips, it may be assumed that such extrusions are viable provided the aspect ratio is less than about 3.0 This limiting value tends to decrease slightly with the stronger types of 6xxx material, or if the tip corners are not rounded It decreases further for the weaker (weldable) forms of 7xxx-series alloy, and much more so for 2xxx, 5xxx and the stronger 7xxx alloys

When a exceeds the limiting value, there are two possible courses.

The first is to extrude the section in an opened-out shape and then roll-form it back to the desired profile during correction (Figure 2.4) The alternative is to extrude it as a quasi-hollow, or ‘semi-hollow’, using a bridge die (see below)

2.3.5 Hollow sections

Hollow sections are normally extruded using a two-piece bridge die

(Figure 2.5) The outside of the section is defined by the aperture in the front part A of the die, and the inside by the mandrel nose on the back

Figure 2.5 Production of hollow section using a bridge die.

Figure 2.4 Roller modification of profile with deep re-entrant.

part B The extrusion speed is a bit less than for a non-hollow profile, leading to a slightly higher cost per kilogram Also, the cost of the die

is likely to be twice as much as for a non-hollow profile, so that a reasonable size of order is needed to make a new section worthwhile Bridge dies are also employed for extrusion of ‘semi-hollow’ profiles, such as that shown in Figure 2.6 For these, the nose on part B of the die defines the area shown shaded in the figure

During the extrusion of a hollow section, the plastic metal has to flow around the support feet of part B and then reunite before emerging The section thus produced therefore contains local zones at which welding has occurred during extrusion, called ‘seam welds’ These are usually

of no consequence and many users do not even know they are there But a designer should be aware of them, since they produce potential lines of weakness down the length of a section which very occasionally cause trouble, especially if the extrusion speed is too high With the section shown in Figure 2.7, to which web-plates are to be welded by the fabricator, there is a risk that transverse shrinkage at these welds might tear the section apart at one of the seam welds, if the latter are located as shown When ordering such sections, it is prudent to tell the extruder where any fabrication welds will be made, so that the seam welds can be located well away from them

Figure 2.6 ‘Semi-hollow’ profile.

Figure 2.7 Seam welds (S) in a hollow profile.